This invention relates to an electronic switch used to disconnect the auxiliary winding of a single-phase electric motor after the motor has started and reached approximately normal running speed. The electronic switch described herein is applicable to single speed, two-speed, and instantly reversible motors, which are the three application conditions commonly required.
Single-phase electric motors inherently have zero torque at standstill. This is because there is primarily a vibrating magnetic field applied to the rotor rather than any rotational torque. A commonly used method for starting single-phase motors is to incorporate an auxiliary winding which is spaced up to 90.degree. electrical degrees away from the main winding and excited with current which leads or lags the exciting current for the main winding. This combination provides a rotating magnetic field in the motor air gap and starting torque results. A commonly used technique for achieving a leading current in the auxiliary winding is to place a capacitor in series with such winding. Since this auxiliary circuit is used only during starting, neither the capacitor not the auxiliary winding is rated for continuous duty, and they are therefore switched out of the circuit as soon as the motor is near its normal running speed. Mechanical centrifugal switches and current-operated relays are two of the motor common techniques used for accomplishing this switching action, with the centrifugal switch being by far the most widely used.
The mechanical switch has a number of disadvantages, including difficulty in adjusting the alignment of the stationary and rotating parts, with improper alignment causing the switch to malfunction.
Another disadvantage is a limited life because of arcing, which causes corrosion and pitting of the electrical contacts. When such mechanical switches are attempted to be used to obtain two-speed operations and instantaneous reversing, such switches become mechanically complex and relatively unreliable. These mechanical shortcomings have been endured for many years because of the low cost of the mechanical centrifugal switches.
An electronic replacement for centrifugal switches has been a long-sought goal which is now realizable in view of the cost reductions being realized as a result of increased circuit integration. Most of the prior art attempts at electronic switching have required a speed feedback signal to the electronic circuit as the means for providing information to the circuit as to when it should trigger the auxiliary switch to the off condition. This added device plus the cost of the electronics have combined to keep the electronic switch from becoming price-competitive with mechanical centrifugal switches. One technique has been to use time as the base for switching, but this technique is satisfactory only for a specific motor and load inertia, and consequently cannot be used for general purpose applications.
U.S. Pat. No. 3,792,324 utilized the voltage across a part of the auxiliary winding and the starting capacitor, or the voltage across the starting capacitor to control the opening of an axuiliary switch. When this switch opened, it terminated current flow through the starting capacitor but the current continued to flow through the starting winding and a running capacitor.
U.S. Pat. No. 3,671,830 rectified a voltage obtained from the auxiliary winding and then used this in a feedback circuit which was variable by means of a rheostat to control a Schmidt trigger circuit controlling the auxiliary switch.
U.S. Pat. No. 3,777,232 utilized a switching signal obtained from the relative phase between the main winding current and the start winding current, or the relative phase between the main winding current and the applied voltage. All of these circuits had limitations effectively limiting the application of such circuits to single-speed motors rather than to the capability of the present circuit, which included dual-speed motor starting circuits and instantaneous reversing circuits.
U.S. Pat. No. 4,443,749 disclosed two and four-pole main windings and an auxiliary winding, with the motor starting on high speed and then changing to low speed if the low speed was selected. This patent utilized a centrifugal switch to de-energize the auxiliary winding. U.S. Pat. No. 4,467,257 has a somewhat similar arrangement.
U.S. Pat. No. 4,622,506 utilized a comparator of the peak detected voltages of the main and auxiliary windings.